Can Water Float on Oil?

The tensile strength of interfaces between air/liquid and immiscible liquid/liquid systems has been the driving force in nature and industrial processes such as wetting/dewetting and coating. Despite its small magnitude, interfacial tension can be the dominant force in capillary tubes, microgravity conditions, and nanofluids. In addition to capillary rise, the surface tension can work against gravity and support a solid body floating in water surface, which has been noticed as early as 350 BC by Aristotle. In nature, insects such as water striders rely on surface tension to walk on water. Experimentally, small spherical particles, few millimeters in diameter, have been reported as floating on the air/water interface. In contrast to rigid bodies, a fluid droplet at surface of another fluid has three deformable interfaces and variable contact angles. The equilibrium of three interface tensions at the contact line, if it exists, results in an unique combination of three contact angles, which form a Neumann’s triangle.The interfacial interaction between three fluids has been investigated for an oil droplet spreading on water surface by Langmuir and for a fluid droplet at the interface between two fluids by Princen.
More recently, numerical methods were applied to identify the shape of such droplets. In these instances, the liquid droplet is less dense than the supporting liquid, and gravity always plays a stabilizing role on the system.

In this work a numerical model was developed from the Young−Laplace equation on three interfaces (water/oil, water/air, and oil/air) to predict the theoretical equilibration conditions. The model was verified successfully with an oil/water system. The stability of the floating droplet depends on the combination of three interface tensions, oil density, and water droplet volume. For practical purposes, however, the equilibrium contact angle has to be greater than 5° so the water droplet can effectively float. This result has significant applications for biodegrading oil wastes.



Can Water Float on Oil?
Chi M. Phan,* Benjamin Allen, Luke B. Peters, Thu N. Le, and Moses O. Tade
Department of Chemical Engineering, Curtin University, GPO Box U1987, Perth WA 6845, Australia


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